Stator and manufacturing method of stator

ABSTRACT

A stator includes a main yoke having a cylindrical shape with a bottom, an auxiliary yoke having a band shape arranged on an outer or inner circumferential wall surface of this main yoke, and a field magnet arranged inside the main yoke. The auxiliary yoke is arranged along a circumferential direction on the outer or inner circumferential wall surface of the main yoke. One end of the auxiliary yoke has at least one protrusion, and another end of the auxiliary yoke has at least one recess facing and engaging in a circumferential direction with the protrusion either on the inner or outer circumferential wall surface of the main yoke.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of InternationalPatent Application No. PCT/JP2017/030221 filed on Aug. 24, 2017, whichdesignated the United States and claims the benefit of priority fromJapanese Patent Applications No. 2016-167804 filed on Aug. 30, 2016, andNo. 2017-110944 filed on Jun. 5, 2017. The entire disclosures of all ofthe above applications are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to a stator and a manufacturing method ofthe stator.

BACKGROUND

A frame structure of a DC motor includes a rotor core enclosed in aframe (yoke) having a cup shape, and an auxiliary frame having a ringshape is arranged on a cylindrical outer surface of this frame.

SUMMARY

According to the present disclosure, a stator includes: a main yokehaving a cylindrical shape with a bottom; an auxiliary yoke having aband shape and arranged on an outer circumferential wall surface or aninner circumferential wall surface of the main yoke; and a field magnetarranged inside the main yoke and radially facing an outer surface ofthe armature. The auxiliary yoke is arranged along a circumferentialdirection of the outer circumferential wall surface or the innercircumferential wall surface of the main yoke. The auxiliary yoke has atleast one protrusion formed at one end thereof. The auxiliary yoke hasat least one recess formed at another end thereof. The recess faces andengages with the protrusion in the circumferential direction in a statewhere the auxiliary yoke is arranged along the circumferential directionof the outer circumferential wall surface or the inner circumferentialwall surface of the main yoke.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of a motor according to afirst embodiment of the present disclosure.

FIG. 2 is a diagram showing a longitudinal cross section of a firststator according to the first embodiment of the present disclosure.

FIG. 3 is a perspective view of the first stator according to the firstembodiment of the present disclosure.

FIG. 4 is a cross-sectional view along line A-A in FIG. 1 and a frontview.

FIG. 5 is illustrative views showing an attachment part of a firstauxiliary yoke according to the first embodiment of the presentdisclosure.

FIG. 6 is an illustrative diagram showing the size configuration of theattachment part of the first auxiliary yoke according to the firstembodiment of the present disclosure.

FIG. 7 is a diagram illustrating a variation example of the attachmentpart of the first auxiliary yoke according to the first embodiment ofthe present disclosure.

FIG. 8 is illustrative views showing a manufacturing process of thefirst stator according to the first embodiment of the presentdisclosure.

FIG. 9 is diagrams showing a variation example of the manufacturingprocess of the first stator according to the first embodiment of thepresent disclosure.

FIG. 10 is a perspective view showing a second stator according to asecond embodiment of the present disclosure.

FIG. 11 is illustrative views showing a rotatable fastening part of asecond auxiliary yoke according to the second embodiment of the presentdisclosure.

FIG. 12 is illustrative views showing the function of the rotatablefastening part according to the second embodiment of the presentdisclosure.

FIG. 13 is a schematic plan view of a split core stator.

FIG. 14 is a perspective view of a wound yoke.

FIG. 15 is illustrative views showing a manufacturing process of thesplit core stator.

FIG. 16 is diagrams illustrating a first variation example of the woundyoke.

FIG. 17 is diagrams illustrating a second variation example of the woundyoke.

DETAILED DESCRIPTION

Hereinafter, multiple embodiments for implementing the presentdisclosure will be described referring to drawings. In the respectiveembodiments, a part that corresponds to a matter described in apreceding embodiment may be assigned the same reference numeral, andredundant explanation for the part may be omitted. When only a part of aconfiguration is described in an embodiment, another precedingembodiment may be applied to the other parts of the configuration. Theparts may be combined even if it is not explicitly described that theparts can be combined. The embodiments may be partially combined even ifit is not explicitly described that the embodiments can be combined,provided there is no harm in the combination. Hereinafter, theembodiments of the present disclosure will be described. Configurationsin the following descriptions do not limit the present disclosure andcan be variously modified within the scope of the present disclosure.

This embodiment describes a stator that can reduce physical impacts on amain yoke when mounting an auxiliary yoke as well as allows easymounting of the auxiliary yoke, and a manufacturing method thereof.

FIG. 1 to FIG. 12 illustrate the present disclosure. FIG. 1 shows aschematic configuration diagram of a motor common to a first embodimentand a second embodiment. FIG. 2 to FIG. 7 illustrate the firstembodiment. FIG. 2 is a diagram showing a longitudinal cross section ofa first stator, FIG. 3 is a perspective view of the first stator, FIG. 4is a cross-sectional view along line A-A in FIG. 1 and a plan view, FIG.5 is illustrative views showing an attachment part of a first auxiliaryyoke, FIG. 6 is an illustrative diagram showing the size configurationof the attachment part of the first auxiliary yoke, FIG. 7 is a diagramillustrating a variation example of the attachment part of the firstauxiliary yoke, FIG. 8 is illustrative views showing a manufacturingprocess of the first stator, and FIG. 9 is diagrams showing a variationexample of the manufacturing process of the first stator. FIG. 2 showsonly the stator and magnets, for illustration of the first stator, anddoes not show other components. FIG. 10 to FIG. 12 illustrate a secondembodiment, FIG. 10 is a perspective view showing a second stator, andFIG. 11 is illustrative views showing a rotatable fastening part of asecond auxiliary yoke. FIG. 12 shows illustrative views for explainingthe function of this rotatable fastening part in detail.

First Embodiment

(Schematic Configuration of Motor)

A motor M shown as an example is a DC motor. The configuration of themotor M will be roughly explained below. The motor M according to thisembodiment is configured by a combination of a rotor 1, a first stator2, an end plate 3, and a brush 4. An output side of the motor M refersto the side to which the power of the motor M is transmitted, which is,in FIG. 1, the left side from the viewer's point. A base side refers tothe opposite side from the output side in the axial direction of arotating shaft 11.

As shown in FIG. 1, the rotor 1 is configured to include the rotatingshaft 11 that is the center of rotation, an armature 12, and acommutator 13. The armature 12 is assembled to the rotating shaft 11such as to be integrally rotatable therewith, and configured to have arotor core 12A, and coils 12B wound around this rotor core 12A. Thecommutator 13 having a cylindrical shape is secured to the rotatingshaft 11. The position where it is secured is on the output side of thearmature 12. The commutator is integrally rotatable with the rotatingshaft 11. The coils 12B that form the armature 12 are electricallyconnected to the commutator 13 (more precisely to a commutator piecebonded to the outer periphery thereof).

The first stator 2 is configured to have a main yoke 21 having a cupshape, a first auxiliary yoke 22 having an annular shape and disposedoutside the main yoke 21, and field magnets 23. A bearing installationpart 21A having a cup shape and protruding toward the base side isformed in the center of a cup-shaped bottom part of the main yoke 21.Parts other than this bearing installation part 21A shall be referred toas “main yoke body part 21B”. This bearing installation part 21A housesa ball bearing K1 having an annular shape therein, and this ball bearingK1 rotatably supports a base-side end of the rotating shaft 11. Thefield magnets 23 are arc-shaped permanent magnets. A plurality ofmagnets (in a number corresponding to the number of poles) are bonded tothe inner wall of the main yoke body part 21B. Since a four-poleconfiguration is illustrated in this example, there are used fourmagnets 23.

The main yoke 21 is a magnetic body having a cup shape (cylindrical witha bottom). The main yoke body part 21B, in particular, plays a role incoupling adjacent magnets 23 bonded on the inner wall with magneticfluxes to configure a magnetic circuit. The first auxiliary yoke 22 is amagnetic body having an annular shape and disposed such as to be woundaround the main yoke body part 21B on the outer surface (outercircumferential wall surface) thereof for enhancing the function of themain yoke 21 as part of the magnetic circuit. The structure and othersof this first auxiliary yoke 22 for attachment to the main yoke 21 arethe main feature of the present disclosure and will be described laterin detail.

The open side of the main yoke 21 is closed by the end plate 3 (brushholder). A through hole (not shown) is formed in a central part of thisend plate 3 for allowing the output side of the rotating shaft 11 toextend through. A ball bearing K2 having an annular shape is disposed onan inner wall surface of this through hole. This ball bearing K2 axiallysupports the output side of the rotating shaft 11 in a rotatable manner.Further, the brush 4 is disposed on a surface of the end plate 3 facingthe base side. This brush 4 is a square columnar member and configuredsuch that its end portion on the radially center side makes contact withan outer surface of the commutator 13 (more precisely, a commutatorpiece bonded to the outer periphery thereof).

As described above, the armature 12 that forms the rotor 1 is housedinside the first stator 2 having a cup shape, and the opening of thefirst stator 2 (open on the output side) is closed by the end plate 3,with the output-side end of the rotating shaft 11 protruding. Thebase-side end and the output-side end of the rotating shaft 11 in thisstate are axially supported by the ball bearings K1 and K2 in arotatable manner, and the brush 4 disposed on the output side of the endplate 3 makes contact with the outer surface of the commutator 13. Thefield magnets 23 are bonded to the inner surface of the main yoke bodypart 21B that forms the first stator 2, and these magnets 23 areconfigured to face the outer surface of the armature 12.

Although not shown, the brush 4 is configured such that current issupplied from an external power supply. The current supplied from thisbrush 4 is commutated by the commutator 13 and supplied to the armature12. The armature 12 that has become an electromagnet with switchingmagnetization directions and the fixed field magnets 23 interact witheach other, which causes the rotor 1 to rotate. This first stator 2 hasone yoke built by a combination of the main yoke 21 and the firstauxiliary yoke 22, which, in this example, is configured such that thefirst auxiliary yoke 22 in an annular shape is arranged on the outersurface of the main yoke 21. While this embodiment describes theconfiguration in which the first auxiliary yoke 22 is arranged on theouter surface of the main yoke 21, it goes without saying that the yokeconfiguration is not limited to this. In an alternative configuration,the first auxiliary yoke 22 in an annular shape may be arranged on aninner surface (inner circumferential wall surface) of the main yoke 21,with the magnets 23 disposed on the inner surface of this firstauxiliary yoke 22. In view of work efficiency during the manufacturing,however, the configuration with the first auxiliary yoke 22 beingdisposed on the outer surface of the main yoke 21 is more preferable.

<Configuration of First Auxiliary Yoke>

The structure of the first auxiliary yoke 22 according to thisembodiment will be described with reference to FIG. 3 to FIG. 7. Thefirst auxiliary yoke 22 according to this embodiment is a cylindricalcomponent formed by rolling a rectangular band-shaped plate-like memberinto a circular shape. The first auxiliary yoke 22 according to thisembodiment is configured to have a first auxiliary yoke body part 22A,first auxiliary yoke protrusions 22B, and first auxiliary yoke recesses22C, as shown in FIG. 3. The first auxiliary yoke body part 22A is arectangular (band-shaped) plate member, which is the part that willbecome cylindrical when it is rolled into a circular shape. For the sakeof explanation, the long side of the first auxiliary yoke body part 22Ain a rectangular shape shall be hereinafter referred to as “long side221”, and the short side as “short side 222”. The long side 221 isformed to have substantially the same length as the length of thecircumference of the outer surface of the main yoke body part 21B.

The first auxiliary yoke protrusions 22B are formed on one short side222 of the first auxiliary yoke body part 22A (i.e., one end of thefirst auxiliary yoke 22). The first auxiliary yoke recesses 22C areformed on the other short side 222 of the first auxiliary yoke body part22A (i.e., the other end of the first auxiliary yoke 22). The firstauxiliary yoke protrusion 22B is a projection protruding from one shortside 222 in a direction along which the long side 221 extends. In thisembodiment, a distal end portion of the first auxiliary yoke protrusion22B is initially formed in a circular arc shape. In the first auxiliaryyoke protrusion 22B, an absorption hole H1 is formed (more precisely,hollowed out), as shown in FIG. 5. As will be described later, thisabsorption hole H1 is a portion that serves as a relief hole when thefirst auxiliary yoke protrusion 22B is deformed inside the firstauxiliary yoke recess 22C. In this embodiment, there are three firstauxiliary yoke protrusions 22B side by side along the axial direction asshown in FIG. 3.

The first auxiliary yoke recess 22C is a portion recessed from the othershort side 222 along the direction in which the long side 221 extends.In this embodiment, the first auxiliary yoke recess 22C is formed tohave an insertion part 223 and a deformation part 224, as shown in FIG.5. The insertion part 223 corresponds to a first hole closer to theopening of the first auxiliary yoke recess 22C, and is a hole in aquadrate shape cut from the other short side 222. The deformation part224 is a second hole farther from the opening of the first auxiliaryyoke recess 22C than the insertion part 223, a quadrate openingcontinuous with the insertion part 223. The axial size of the insertionpart 223 (opening width) is set substantially equal to the axial size(length in the axial direction) of a base part of the first auxiliaryyoke protrusion 22B and smaller than the axial size of the deformationpart 224. Namely, the first auxiliary yoke recess 22C is a slit-likehole opened along a direction in which the long side 221 extends (openedfrom one short side 222 toward the other short side 222), with itsentrance side (on the end side of one short 222) being a narrower hole(insertion part 223) and the deeper side being a wider hole (deformationpart 224). More specifically, as shown in FIG. 5, there is formed a step225 in the shape of letter L between the insertion part 223 and thedeformation part 224. In this embodiment, as shown in FIG. 3, threefirst auxiliary yoke recesses 22C are formed side by side in the axialdirection. The positions of the three first auxiliary yoke protrusions22B are determined such that, when the first auxiliary yoke body part22A is rolled into a circular shape so that both short sides 222, 222meet, the three first auxiliary yoke protrusions 22B are aligned withthe positions of the three first auxiliary yoke recesses 22C.

In this embodiment, as shown in FIG. 3, buffer holes H2 are formed nearthe base side of the first auxiliary yoke protrusions 22B and near thedeformation parts 224 of the first auxiliary yoke recesses 22C. Thesebuffer holes H2 are portions that provide buffer for preventing a forceapplied by an engagement operation in which the first auxiliary yokeprotrusion 22B is engaged with the first auxiliary yoke recess 22C, anddeformation caused by this force, from propagating to other parts of thefirst auxiliary yoke body part 22A (parts other than the ends where theengagement takes place). Namely, when the first auxiliary yoke body part22A is rolled into a circular shape so that both short sides 222, 222meet, the distal ends of the first auxiliary yoke protrusions 22B fitinto the first auxiliary yoke recesses 22C, causing the absorption holesH1 to deform. With both short sides 222, 222 making tight contact witheach other, when the distal ends of the first auxiliary yoke protrusions22B are pushed deeper into the first auxiliary yoke recesses 22C, thebuffer holes H2 deform to exhibit their function.

In this embodiment, as shown in FIG. 4, the joint of the first auxiliaryyoke 22, i.e., where one short side 222 is caused to abut on the othershort side 222, is arranged outside the position where the magnet 23 isarranged. Preferably, this abutting part is positioned within acircumferential range where the magnet 23 is arranged, as shown in FIG.4(b). Even more preferably, the position where the first auxiliary yokeprotrusion 22B engages with the first auxiliary yoke recess 22C isradially aligned with the center of gravity of the magnet 23. Sincethree first auxiliary yoke protrusions 22B engage with three firstauxiliary yoke recesses 22C in this embodiment, the position where thefirst auxiliary yoke protrusion 22B and the first auxiliary yoke recess22C located at the center in the axial direction engage with each otheris arranged to be radially aligned with the position which is the centerof gravity of the magnet 23. When there is one position where the firstauxiliary yoke protrusion 22B engages with the first auxiliary yokerecess 22C, this single engagement position should preferably beradially aligned with the position which is the center of gravity of themagnet 23. The point in question of the magnet 23 is a portion that isnot used as a magnetic path and is not affected by a magnetic loss.Therefore, this position in question is made to be one of the engagementpositions between the first auxiliary yoke protrusions 22B and the firstauxiliary yoke recesses 22C.

Next, the engagement between the first auxiliary yoke protrusion 22B andthe first auxiliary yoke recess 22C will be described with reference toFIG. 5 to FIG. 7. As shown in FIG. 5(a), the first auxiliary yokeprotrusion 22B is inserted into the first auxiliary yoke recess 22C. Asshown in FIG. 6, the first auxiliary yoke protrusion 22B is configuredto have a length t2 (distance in the direction in which the long side221 extends) slightly larger than the length t1 of the first auxiliaryyoke recess 22C in the same direction. Therefore, when the distal endportion of the first auxiliary yoke protrusion 22B abuts on the bottomside of the first auxiliary yoke recess 22C, a small gap K is formedbetween one short side 222 and the other short side 222 as shown in FIG.5(b). This gap K has a width Δt that equals to Δt2−Δt1. As shown in FIG.6, the insertion part 223 is formed to have an axial size t3 that issubstantially equal to the axial size t4 of the first auxiliary yokeprotrusion 22B (length t3≈length t4). Because of this configuration, inthe state shown in FIG. 5(b), the base side of the first auxiliary yokeprotrusion 22B is retained in the insertion part 223. In thisembodiment, in order to engage the first auxiliary yoke protrusion 22Bmore firmly with the first auxiliary yoke recess 22C, a force is furtherapplied in the direction of the arrow from the state shown in FIG. 5(b).Needless to say, from the viewpoint of facilitating insertion of thefirst auxiliary yoke protrusion 22B into the insertion part 223, theaxial size t3 of the insertion part 223 may be made slightly larger thanthe axial size t4 of the first auxiliary yoke protrusion 22B.

In this way, as a force is applied in the direction of the arrow in thestate shown in FIG. 5(b), the width of the gap K becomes almost 0, sothat one short side 222 and the other short side 222 make contact(including pressure contact) with or come closer to each other.Simultaneously, the distal end of the first auxiliary yoke protrusion22B makes pressure contact with the first auxiliary yoke recess 22C, sothat, as shown in FIG. 5(c), the distal end portion of the firstauxiliary yoke protrusion 22B undergoes deformation inside thedeformation part 224 of the first auxiliary yoke recess 22C. Since thedeformation part 224 is formed to have an axial size t5 that is largerthan the axial size t4 of the first auxiliary yoke protrusion 22B, thisdifference in distance provides allowance for deformation, so that thedistal end of the first auxiliary yoke protrusion 22B undergoesdeformation inside the first auxiliary yoke recess 22C. In other words,the distal end of the first auxiliary yoke protrusion 22B is compressed.The absorption hole H1 formed in the distal end portion of the firstauxiliary yoke protrusion 22B deforms at this time, thereby effectivelypreventing the first auxiliary yoke protrusion 22B from breaking causedby the applied force.

As described above, in this embodiment, the distal end portion of thefirst auxiliary yoke protrusion 22B is compressed inside the deformationpart 224 such as to spread in the axial direction, as a result of whichthe axial size of the distal end portion of the first auxiliary yokeprotrusion 22B becomes larger than the axial size t3 of the insertionpart 223. In other words, a spread part 226 is formed at the distal endof the first auxiliary yoke protrusion 22B as a result of this distalend portion being compressed to spread so that its axial size isprolonged. Both axial ends of this spread part 226 make pressure contactwith the edge surfaces of the deformation part 224 positioned at bothaxial ends as shown in FIG. 5(c), and thus the first auxiliary yokeprotrusion 22B engages with the first auxiliary yoke recess 22C. Thisenables a reliable and firm engagement between the first auxiliary yokeprotrusion 22B and the first auxiliary yoke recess 22C. With the distalend of the first auxiliary yoke protrusion 22B making pressure contactwith the inside of the deformation part 224 as described above, thefirst auxiliary yoke protrusion 22B is effectively prevented from comingout of the insertion part 223.

As has been described above, according to the first embodiment of thepresent disclosure, with the first auxiliary yoke 22 being arranged onthe outer surface of the main yoke 21 along the circumferentialdirection thereof, the first auxiliary yoke protrusions 22B face andengage with the first auxiliary yoke recesses 22C. Thus, the firstauxiliary yoke 22 can be wound around the outer surface of the main yoke21 without affecting (more specifically, without causing deformation inthe inner diameter of) the main yoke 21. The accuracy requirements forthe inner and outer diameters of the main yoke 21 and the firstauxiliary yoke 22 can be made less stringent, and problems such aspeeling of coating or machine oil accumulation on the outer surface ofthe main yoke 21 can be made less likely to occur. The engagementstructure described above applies similarly to a configuration whereinthe first auxiliary yoke 22 is arranged along the inner surface of themain yoke 21. In such a configuration, the first auxiliary yokeprotrusion 22B is engaged with the first auxiliary yoke recess 22C onthe radially center side relative to the inner surface of the main yoke21.

When both axial ends of the spread part 226 formed at the distal end ofthe first auxiliary yoke protrusion 22B make pressure contact with theedge surfaces positioned at both axial ends of the deformation part 224as shown in FIG. 5(c), the engagement is maintained only by frictiongenerated therebetween. If, as shown in FIG. 7, the first auxiliary yokeprotrusion 22B engages with the first auxiliary yoke recess 22C suchthat the surface of the spread part 226 positioned at the base side ofthe first auxiliary yoke protrusion 22B makes tight contact with(tightly abutting on) the step 225 in the first auxiliary yoke recess22C, the engagement will be maintained even more firmly.

<Manufacturing Method of First Stator>

Next, a manufacturing method of the first stator 2 according to thisembodiment will be described with reference to FIG. 8. As has beendescribed above and shown in FIG. 8(a), the first auxiliary yoke 22 isinitially formed as a rectangular band-shaped plate member. Three firstauxiliary yoke protrusions 22B are formed on one short side, while threefirst auxiliary yoke recesses 22C are formed on the other short side. Ina placement step shown in FIG. 8(b), the first auxiliary yoke body part22A of this first auxiliary yoke 22 having a band shape is wound aroundthe outer surface of the main yoke body part 21B. The winding is carriedout such that the first auxiliary yoke protrusions 22B and the firstauxiliary yoke recesses 22C abut on each other in the circumferentialdirection. Then, in an insertion step, as shown by the arrow in FIG.8(b), the first auxiliary yoke protrusions 22B are inserted into thefirst auxiliary yoke recesses 22C (see also FIG. 5A). Next, in apressing step shown in FIG. 8(c), a force F is further applied in thecircumferential direction to press the distal ends of the firstauxiliary yoke protrusions 22B to the inner edges of the deformationparts 224 of the first auxiliary yoke recesses 22C to deform the distalends of the first auxiliary yoke protrusions 22B. Finally, distal endportions of the first auxiliary yoke protrusions 22B undergo deformation(in other words, form spread parts 226) inside the deformation parts 224of the first auxiliary yoke recesses 22C to make pressure contact withthe edge surfaces at both axial ends of the deformation parts 224. Thiscauses the first auxiliary yoke protrusions 22B to engage with the firstauxiliary yoke recesses 22C reliably and firmly (see also FIG. 5(b) andFIG. 5(c)). The first auxiliary yoke 22 is thus attached to the mainyoke 21. If the surfaces of the spread parts 226 positioned on the baseside of the first auxiliary yoke protrusions 22B are in tight contactwith (tightly abutting on) the steps 225 between the insertion parts 223and the deformation parts 224, the engaging state of the first auxiliaryyoke protrusions 22B and the first auxiliary yoke recesses 22C can bemaintained even more firmly.

In forming the first stator 2, a step of placing the magnets 23 is alsoperformed, which may be carried out at any stage. Preferably, themagnets should be placed prior to the winding of the first auxiliaryyoke 22 around the main yoke body part 21B in order that the positionwhere the first auxiliary yoke protrusions 22B and the first auxiliaryyoke recesses 22C are to engage with each other can be more readilydetermined. Since the first auxiliary yoke 22 is wound around the outersurface of the main yoke body part 21B in this embodiment, the magnets23 are disposed on the inner surface of the main yoke body part 21B byany method such as adhesion using an adhesive, welding, and the like.

<Variation Example of the Manufacturing Method of First Stator>

Next, a variation example of the manufacturing method of the firststator 2 according to this embodiment will be described with referenceto FIG. 9. Although the steps shown in FIG. 9(c) to FIG. 9(e) arecarried out using two split molds S1 and S2 to be described later, thesplit molds S1 and S2 are omitted in FIG. 9(c) to FIG. 9(e) forconvenience of explanation. FIG. 9(c) to FIG. 9(e) also show an enlargedview of the attachment portion of the first auxiliary yoke 22. Accordingto the variation example of the manufacturing method of the first stator2, the main yoke 21 and the first auxiliary yoke 22 are sandwichedbetween forming molds (split molds S1 and S2) split into two, upper andlower, parts as shown in FIG. 9(a), and both yokes are radiallycompressed to set the first auxiliary yoke 22 on the outer surface ofthe main yoke 21. More specifically, in the placement step, the firstauxiliary yoke body part 22A of the first auxiliary yoke 22 having aband shape is wound around the outer surface of the main yoke body part21B. After that, the main yoke 21 with the first auxiliary yoke 22 woundtherearound is encased inside an enclosure space of substantially acolumnar shape formed between the two split molds S1 and S2 as shown inFIG. 9(b). With the main yoke 21 and the first auxiliary yoke 22 beingencased inside the enclosure space, both yokes sandwiched between thetwo split molds S1 and S2 are radially compressed, to carry out theinsertion step and pressing step. As a result, the distal end of thefirst auxiliary yoke protrusion 22B is inserted through the insertionpart 223 into the first auxiliary yoke recess 22C, makes pressurecontact with the deformation part 224 and deforms, whereby the spreadpart 226 is formed, as shown in FIG. 9(c). After that, when the mainyoke 21 and the first auxiliary yoke 22 are further compressed radially,both yokes reduce in diameter as shown in FIG. 9(d), and the distal endof the first auxiliary yoke protrusion 22B is further compressed so thatthe spread part 226 expands to a length corresponding to the axial sizeof the deformation part 224. After the steps described above, while themain yoke 21 tries to restore to its original diameter, the firstauxiliary yoke 22 tries to keep its compressed state against thepressure from the main yoke 21. Thus, the first auxiliary yokeprotrusion 22B is subjected to a force that causes it to be releasedfrom the first auxiliary yoke recess 22C. The spread part 226, however,is hooked on the step 225 between the insertion part 223 and thedeformation part 224 as shown in FIG. 9(e). Thus, the engaging statebetween the first auxiliary yoke protrusion 22B and the first auxiliaryyoke recess 22C is maintained firmly.

Second Embodiment

Next, a second embodiment will be described with reference to FIG. 10 toFIG. 12. The second stator 102 according to this embodiment, as comparedto the first embodiment described above, has a second auxiliary yoke 6having a different shape from that of the first auxiliary yoke 22, andis similar in other features. The second auxiliary yoke 6 includes asecond auxiliary yoke body part 6A and a rotatable fastening part 6B asshown in FIG. 10. The second auxiliary yoke body part 6A is arectangular (band-shaped) plate member, which is the part that willbecome cylindrical when it is rolled into a circular shape. For the sakeof explanation, the long side of the second auxiliary yoke body part 6Ain a rectangular shape shall be hereinafter referred to as “long side106”, and the short side as “short side 206”. This long side 106 isformed slightly shorter than the length of the circumference of theouter surface of the main yoke body part 21B.

The rotatable fastening part 6B is formed to have an action part 61, oneconnecting part 62, and the other connecting part 63, as shown in FIG.10. The action part 61 is formed in a quadrate plate-like shape. Oneconnecting part 62 extends from one point P1 at one point in thecircumference thereof toward one short side 206, while the otherconnecting part 63 extends from another point P2 at another point in thecircumference thereof toward the other short side 206. The one point P1and the other point P2 are set at point symmetrical positions about thecenter of the action part 61. Because of this configuration, applying atorque in the direction of black arrows to the action part 61 causes thecircumferential distance between the short sides 206, 206 of the secondauxiliary yoke body part 6A to reduce.

Namely, as shown in FIG. 11, applying a torque in the direction of blackarrows to the action part 61 causes the circumferential distance betweenthe short sides 206, 206 of the second auxiliary yoke body part 6A toreduce from t6 to t7. Thus, the second auxiliary yoke 6 can be attachedto the outer circumference of the main yoke body part 21B by insertingthe main yoke body part 21B into the second auxiliary yoke 6 in aninitial state where the second auxiliary yoke 6 is formed to have alarger inner circumference than the outer circumference of the main yokebody part 21B by (t6−t7), and by applying a torque in the direction ofblack arrows to the action part 61. FIG. 12 schematically shows thefunction of the rotatable fastening part 6B. By a series of actions asshown in FIGS. 12(a), 12(b), and 12(c), the circumferential distancebetween the short sides 206, 206 of the second auxiliary yoke body part6A is reduced from t6 to t7 as described above. By applying a torque inthe opposite direction to the action part 61, the circumferentialdistance between the short sides 206, 206 of the second auxiliary yokebody part 6A can be widened from t6 to t8. Thus, in this embodiment, thesecond auxiliary yoke 6 is detachable, and fine adjustments can also beeasily made.

When this second auxiliary yoke 6 is to be mounted to the innercircumferential surface of the main yoke body part 21B, thecircumferential distance between the short sides 206, 206 of the secondauxiliary yoke body part 6A can be widened from t6 to t8. This isachieved by a series of actions as shown in FIGS. 12(a), 12(d), and12(e). That is, the second auxiliary yoke 6 can be attached to the innercircumference of the main yoke body part 21B by inserting the secondauxiliary yoke 6 into the main yoke body part 21B in an initial statewhere the second auxiliary yoke 6 is formed to have a smaller innercircumference than the inner circumference of the main yoke body part21B by (t8−t6), and by applying a torque in the direction of blackarrows in FIG. 12(d) to the action part 61.

The rotatable fastening part 6B should also preferably be arranged onthe outer side of a position where the magnet 23 is arranged for similarreasons as the first embodiment. The following is the manufacturingmethod of the second stator 102. First, a placement step is performed,in which the main yoke body part 21B is inserted into the secondauxiliary yoke 6 in an initial state (where it is cylindrical, with theshort sides 206, 206 being coupled together via the rotatable fasteningpart 6B) (alternatively, the second auxiliary yoke 6 is inserted intothe main yoke body part 21B). Next, a pressing step is performed, inwhich the action part 61 is rotated to reduce the circumferentialdistance between the short sides 206, 206 of the second auxiliary yokebody part 6A (or spread them apart) to cause the second auxiliary yoke 6to make pressure contact with the outer circumferential wall surface (orinner circumferential wall surface) of the main yoke body part 21B.Other steps such as setting magnets 23 in making the second stator 102are similar to those of the first embodiment and will not be describedagain.

<Manufacturing Method of Split Core Stator>

Next, a manufacturing method of a split core stator 7, which is oneapplication example of the stator manufacturing method described above,will be described with reference to FIG. 13 to FIG. 17. FIG. 13 is aschematic plan view of the split core stator 7, FIG. 14 is a perspectiveview of a wound yoke 72, FIGS. 15(a) and 15(b) are illustrative viewsshowing a manufacturing process of the split core stator 7, and FIGS.16(a), 16(b), 17(a), and 17(b) are diagrams illustrating variationexamples of a wound yoke 72. While FIG. 14 shows the wound yoke 72 in astate wound around the outer surface of a split core 71, the drawingdoes not show the split core 71 for convenience of illustration.

The split core stator 7 is configured to include a split core 71 in acircular shape, and the wound yoke 72, as shown in FIG. 13. The splitcore 71 is formed by core pieces 71A substantially in the shape ofletter T circumferentially aligned in a ring shape. The wound yoke 72 isa circular metal plate arranged around the outer surface of the splitcore 71. As a comparative example, the split core stator 7 could beconfigured by press-fitting the wound yoke 72 pre-formed in acylindrical shape to the split core 71 that has been temporarilyassembled from core pieces 71A circumferentially arranged in a ringshape. However, this procedure entails a risk that the split core 71 maycollapse (more precisely, the core pieces 71A may be disjointed) whenthe wound yoke 72 is press-fitted to the temporarily assembled splitcore 71. Adopting a structure similar to the first auxiliary yoke 22 orsecond auxiliary yoke 6 described above in the wound yoke 72 will alloweasy attachment of the wound yoke 72 around the outer surface of thetemporarily assembled split core 71, which in turn will enable easyassembling of the split core stator 7.

More specifically, the wound yoke 72 having the same structure as thatof the first auxiliary yoke 22 will have a wound yoke body part 72A in aband shape, with a protrusion 72B at one end and a recess 72C at theother end, as shown in FIG. 14. The wound yoke body part 72A has thesame structure as that of the first auxiliary yoke body part 22Adescribed above, and is configured such that its long sides havesubstantially the same length as the circumferential length (length inthe circumferential direction) of the outer surface of the split core71. The protrusion 72B has the same structure as that of the firstauxiliary yoke protrusion 22B described above, and the recess 72C hasthe same structure as that of the first auxiliary yoke recess 22Cdescribed above.

A wound yoke 72 having the configuration as described above can bearranged on the outer surface of the split core 71 by substantially thesame procedure as that by which the first auxiliary yoke 22 is arrangedon the outer surface of the main yoke 21 in the first embodiment. Morespecifically, first, the split core 71 is temporarily assembled, byarranging the core pieces 71A along the circumferential direction in aring shape as shown in FIG. 15(a). The core pieces 71A can be readilyarranged in a circular shape by setting a jig T having a columnar shapein the center of the core (at the position where it contacts the innersurfaces of the core pieces 71A), and arranging the core pieces 71Aaround the outer circumferential surface of this jig T. After that, thewound yoke body part 72A of the wound yoke 72 is wound around the outersurface of the temporarily assembled split core 71 as shown in FIG.15(b). With the above-mentioned jig T maintained in the core center, thewound yoke 72 can be wound around while circularity of the split core 71is kept favorably.

Once the wound yoke 72 is wound around the outer surface of the splitcore 71, the protrusion 72B and the recess 72C abut on each other in thecircumferential direction. The insertion step is performed once thisstate is achieved, in which the wound yoke 72 is pulled radially towardthe center to insert the protrusion 72B into the recess 72C. This causesthe distal end of the protrusion 72B to pass through the insertion part223 and enter the deformation part 224 of the recess 72C in the sameprocedure as that shown in FIG. 5A. A pressing step is performedthereafter, wherein the distal end of the protrusion 72B is pressedagainst the inner edges of the deformation part 224 of the recess 72C tocompress the distal end of the protrusion 72B in the same procedure asthat shown in FIG. 5(b). In this way, similarly to the situation shownin FIG. 5(c), the distal end of the protrusion 72B is deformed insidethe deformation part 224 of the recess 72C and forms the spread part226, which makes pressure contact with the edge surfaces at both axialends of the deformation part 224. Thus, the protrusion 72B is engagedwith the recess 72C reliably and firmly. The wound yoke 72 is assembledto the split core 71 this way. In a compression step, the wound yokebody part 72A is pulled radially toward the center, so that the corepieces 71A positioned inside the wound yoke body part 72A are pressedradially toward the center. Therefore, each of the core pieces 71A ispressed against the outer circumferential surface of the jig T having acolumnar shape, and the circularity of the split core 71 canconsequently be improved even more.

The respective shapes of the protrusion 72B and the recess 72C are notlimited to those similar to the shapes of the first auxiliary yokeprotrusion 22B and the first auxiliary yoke recess 22C of the firstembodiment and may have other shapes. To give one example, theprotrusion 72B and the recess 72C may engage with each other by asnap-fit configuration as shown in FIG. 16. Namely, both axial ends atthe distal end of the protrusion 72B may protrude in a pawl shape,whereas the insertion part 223 of the recess 72C may have a taperedshape corresponding to the distal end shape of the protrusion 72B, asshown in FIG. 16(a). With this configuration, when the wound yoke 72 ispulled radially toward the center to insert the protrusion 72B into therecess 72C, the distal end of the protrusion 72B pushes apart theinsertion part 223 of the recess 72C as it enters into the deformationpart 224 of the recess 72C, after which the pushed-apart insertion part223 returns to its original size as shown in FIG. 16(b). Thus, theprotrusion 72B engages with the recess 72C by a snap-fit configuration.

The method of assembling the wound yoke 72 to the split core 71 is notlimited to winding a wound yoke body part 72A having a band-shape aroundthe outer surface of the split core 71. The method shown in FIG. 17 mayalso be used. In the method shown in FIG. 17, the wound yoke 72 isassembled to the split core 71 such that the split core 71 is inserted(more precisely, loosely fitted) into the wound yoke body part 72A thathas been rolled into a cylindrical shape in advance, after which theinner diameter of the wound yoke body part 72A is reduced to be equal tothe diameter of the outer surface of the split core 71.

More specifically, in the method shown in FIG. 17, the wound yoke 72 hasa gap section 72G at a circumferential location of the wound yoke bodypart 72A in a cylindrical shape as shown in FIG. 17(a), this gap sectionincluding a first extension 72D, a second extension 72E, and a centerconnector 72F. The first extension 72D is a quadrate portion extendingfrom one circumferential end to the other end of the gap section 72G.The second extension 72E is a quadrate portion extending from the othercircumferential end to the one end of the gap section 72G. The firstextension 72D and second extension 72E are arranged in symmetry, axiallyseparated from each other, and positioned such as to circumferentiallypartly overlap each other. There is a clearance Q each between thedistal end of the first extension 72D and the other circumferential endof the gap section 72G, and between the distal end of the secondextension 72E and the one circumferential end of the gap section 72G.The widths (lengths in the circumferential direction) of these twoclearances Q are the same. When attaching the wound yoke 72 to the splitcore 71, the inner diameter of the wound yoke body part 72A is reducedby a length corresponding to the clearances Q as shown in FIG. 17(b).Namely, as the clearances Q are removed, the circumferential length ofthe gap section 72G is shortened, and the inner diameter of the woundyoke body part 72A is reduced by that amount.

The center connector 72F is present between the first extension 72D andthe second extension 72E in the axial direction and extend long alongthe axial direction. This center connector 72F is substantiallyrectangular as viewed from the side as shown in FIG. 17(a) before thewound yoke 72 is assembled to the split core 71. When the diameter ofthe wound yoke body part 72A is reduced to attach the wound yoke 72 tothe split core 71, the center connector 72F undergoes deformation(deflects) such that one side at one axial end of the center connector72F is shifted relative to the side at the other end as shown in FIG.17(b). In other words, the center connector 72F deforming from the stateshown in FIG. 17(a) to the state shown in FIG. 17(b) causes the diameterof the wound yoke body part 72A to be reduced by an amount correspondingto the clearances Q mentioned above. A wound yoke 72 having theconfiguration described above can easily be attached to a temporarilyassembled split core 71 (without collapsing the split core 71).

A comparative example will be described below. According to thecomparative example, a frame structure of a DC motor includes a rotorcore enclosed in a frame having a cup shape (yoke), and an auxiliaryframe having a ring shape is arranged on the cylindrical outer surfaceof this frame (yoke). This configuration allows formation of a framehaving a small thickness as a whole and yet having a necessary thicknessin a section where a larger thickness is required for forming a magneticcircuit, with an auxiliary frame (auxiliary yoke) wound around thissection.

The technique of the comparative example thus allows the thickness to beincreased only in a section where it is necessary as part of a magneticcircuit while keeping the thickness of other parts small, by using anauxiliary frame (auxiliary yoke). This enables an attempt to reduce thematerial cost and the weight of the frame (yoke). In such technique, theauxiliary frame (hereinafter referred to as “auxiliary yoke”) is fixedlyfitted to a frame having a cup shape (hereinafter referred to as “mainyoke”) by press-fitting or bonding.

However, according to an investigation conducted by the inventors, themethod of attaching an auxiliary yoke to a main yoke by press-fittingcauses deformation in the main yoke by the force applied during thepress-fitting, resulting in a change in inner diameter of the main yoke.The force applied during press-fitting also causes the coating on themain yoke or auxiliary yoke to peel. Moreover, for the press-fitting ofthe auxiliary yoke, the inner and outer diameters of the main yoke needto be finished with high precision, which leads to high manufacturingcost. The auxiliary yoke is also required to have precise inner andouter diameters, which increases the manufacturing cost. The method ofattaching the auxiliary yoke to the main yoke by bonding entails a riskof the auxiliary yoke coming off due to an insufficient bonding force.Moreover, stray adhesive may cause deterioration in appearance. Otherpossible mounting methods include welding or drawing. The former entailsrisks of a change in inner diameter of the main yoke due to thermaleffects, or corrosion of welded spots. The latter has risks of loweredprecision of the main yoke inner diameter caused by anisotropy of thematerial, and of increased difficulty in positioning the blank with highprecision. The latter method also entails a problem of machine oilaccumulation between the auxiliary yoke and the main yoke. Under thesecircumstances, the development of a technique that does not cause achange in the inner diameter of the main yoke, or require too high alevel of precision for the inner and outer diameters, has been soughtafter.

In contrast to the comparative example, the present disclosure providesa stator configured to reduce the influence of attachment of anauxiliary yoke on a main yoke when the auxiliary yoke is attached to themain yoke, and provides a manufacturing method of the stator.

The present disclosure also provides a stator that allows a reduction ofaccuracy requirements for inner and outer diameters of the main yoke andauxiliary yoke and advantageous in terms of manufacturing cost, andprovides a manufacturing method of the stator.

According to the present disclosure, a stator has a cylindrical shapewith a bottom, forms a rotating electric machine, and houses an armaturesecured to a rotating shaft. The stator includes: a main yoke having acylindrical shape with a bottom; an auxiliary yoke having a band shapeand arranged on an outer circumferential wall surface or an innercircumferential wall surface of the main yoke; and a field magnetarranged inside the main yoke and radially facing an outer surface ofthe armature. The auxiliary yoke is arranged along a circumferentialdirection of the outer circumferential wall surface or the innercircumferential wall surface of the main yoke. The auxiliary yoke has atleast one protrusion formed at one end thereof. The auxiliary yoke hasat least one recess formed at another end thereof. The recess faces andengages with the protrusion in the circumferential direction in a statewhere the auxiliary yoke is arranged along the circumferential directionof the outer circumferential wall surface or the inner circumferentialwall surface of the main yoke.

In the present disclosure, the auxiliary yoke is configured such that itis arranged along the circumferential direction of the outercircumferential wall surface or inner circumferential wall surface ofthe main yoke, with the protrusion and recess respectively formed at onecircumferential end and the other circumferential end of the auxiliaryyoke being caused to circumferentially abutting on and engaged with eachother.

Therefore, when setting the auxiliary yoke, no large physical force isexerted as would be by press-fitting or the like, so that adverseeffects to the main yoke (such as changes in inner diameter, peeling ofcoating) can be prevented effectively. Adverse effects by a chemicalforce caused by welding (such as thermal denaturation, corrosion,changes caused by anisotropy) can also be prevented effectively, andthere will be no risk of oil accumulation that would be caused bydrawing. There is no risk of separation which could occur duringbonding, or deterioration in appearance, either. The engagingconfiguration is advantageous in terms of manufacturing cost because itallows reduction of the accuracy requirements for the inner and outerdiameters of the main yoke and auxiliary yoke.

In this case, for more specific configuration, the protrusion may engagewith the recess in a state where the protrusion is in pressure contactwith a part of the recess. The engaging stiffness is enhanced.

Further, for more specific configuration, the recess may include a firsthole close to an opening of the recess, and a second hole which isspaced more from the opening than the first hole is from the opening,the second hole communicating with the first hole. A size of the firsthole in an axial direction may be smaller than a size of the second holein the axial direction. A distal end portion of the protrusion may becompressed and have a spread part expanded in the axial direction. Theprotrusion may engage with the recess in a state where a surface of thespread part facing a base of the protrusion is in contact with a stepformed between the first hole and the second hole. This configurationcauses the protrusion to firmly engage with the recess and allows foreffective prevention of separation of the protrusion from the recess.

Further, for more specific configuration, a size of an opening of therecess in an axial direction may be smaller than a size of an inner partof the recess in the axial direction. A distal-end side of theprotrusion may be positioned in the inner part of the recess, and adistal end of the protrusion may be in pressure contact with a part of aperipheral edge defining the inner part of the recess. A size of a baseside of the protrusion in the axial direction may be smaller than a sizeof the distal-end side of the protrusion in the axial direction, and thebase side may be positioned in the opening of the recess. Thisconfiguration allows for effective prevention of separation of theprotrusion from the recess and ensures effective engagement afterengaging.

When a buffer hole is formed in at least one of a vicinity of theprotrusion and a vicinity of the recess, the buffer hole can be used asa relief hole for relieving an engaging force exerted from theprotrusion engaging with the recess, so that the influence of theengaging force on other parts can be reduced.

According to the present disclosure, a stator has a cylindrical shapewith a bottom, forms a rotating electric machine, and houses an armaturesecured to a rotating shaft. The stator includes: a main yoke having acylindrical shape with a bottom; an auxiliary yoke arranged on an outercircumferential wall surface or an inner circumferential wall surface ofthe main yoke, the auxiliary yoke including an auxiliary yoke body partthat is a band-shaped plate member, and having a cylindrical shape byone end and another end of the auxiliary yoke body part being coupledtogether; and a field magnet arranged inside the main yoke and radiallyfacing an outer surface of the armature. The auxiliary yoke is arrangedalong a circumferential direction of the outer circumferential wallsurface or the inner circumferential wall surface of the main yoke. Theone end and the other end of the auxiliary yoke body part are coupledtogether in a circumferential direction via a rotatable fastening partwhich is rotatable to cause one of the ends to come close to or moveaway from another of the ends. The auxiliary yoke is in pressure contactwith the inner circumferential wall surface or the outer circumferentialwall surface of the main yoke.

In this case, for more specific configuration, the rotatable fasteningpart may include: an action part having a flat plate shape; a one-sideconnecting part that connects one point of the action part with the oneend of the auxiliary yoke body part; and an other-side connecting partthat connects another point of the action part with the other end of theauxiliary yoke body part. The one and other points may be arranged atpoint symmetrical positions about a center of the action part. With thisconfiguration, the auxiliary yoke can be mounted to the main yoke onlyby rotating the rotatable fastening part. Various advantageous effectssimilar to the engagement between the protrusion and the recess can beachieved.

Further, for more specific configuration, the protrusion and the recess,or the rotatable fastening part may be provided at or near a positionradially overlapping with a position of the center of gravity of thefield magnet. This region of the field magnet does not contribute as amagnetic passage. The engaging portions of the protrusion and therecess, or the rotatable fastening part may be positioned at thisregion. Accordingly, adverse influence of a magnetic loss can beprevented.

A manufacturing method of a stator, according to the present disclosure,is a method for manufacturing a stator configured to include: a mainyoke having a cylindrical shape with a bottom; an auxiliary yoke havinga band shape and arranged on an outer circumferential wall surface or aninner circumferential wall surface of the main yoke; and a field magnetarranged inside the main yoke and radially facing an outer surface of anarmature. The stator houses the armature secured to a rotating shaft.The method includes a placing step, an inserting step and a pressingstep. The placing step includes rolling the auxiliary yoke having a bandshape and including at least one protrusion at one end thereof and atleast one recess at another end thereof to engage with the protrusion.The auxiliary yoke is rolled along the outer circumferential wallsurface or the inner circumferential wall surface of the main yoke sothat the protrusion and the recess abut on each other along acircumferential direction of the outer circumferential wall surface orthe inner circumferential wall surface of the main yoke. The insertingstep includes inserting the protrusion into the recess. The pressingstep includes engaging the protrusion with the recess via deformation ofthe protrusion by pressing a distal end portion of the protrusionagainst a circumferential end portion that is a part of the recess.

In this case, the inserting step and the pressing step may be performedin a state where the main yoke and the auxiliary yoke wound around theouter circumferential wall surface of the main yoke are housed in anenclosure space formed between two split molds. The inserting of theprotrusion into the recess and the engaging of the protrusion with therecess may be performed by the two split molds sandwiching andcompressing the main yoke and the auxiliary yoke in a radial directionof the main yoke in the enclosure space. Accordingly, the auxiliary yokecan readily be wound around the outer circumferential wall surface ofthe main yoke, and the protrusion can be readily engaged with therecess, by radially compressing the main yoke while sandwiching the mainyoke and the auxiliary yoke between the two split molds.

In the above-described method, an absorption hole may be formed in theprotrusion, and the engaging of the protrusion with the recess at thepressing step may include deforming the absorption hole to deform thedistal end portion of the protrusion in the recess.

The auxiliary yoke having a band shape is thus configured such that itis mounted by being wound around so that the protrusion and the recessare caused to circumferentially abut on and engaged with each other.Similar advantageous effects as those described above can thus beachieved.

In the pressing step, the distal end of the protrusion is brought intopressure contact with the recess, and engaged with the recess, whiledeforming (compressing) the recess. Thus they can be engaged with eachother only by applying a force in a circumferential direction. Since theprotrusion has deformed (compressed) inside the recess after thepressing step, the protrusion is effectively prevented from separatingfrom the recess. The protrusion can thus be engaged with the recesseasily and reliably.

Since an absorption hole is formed in the distal end portion of theprotrusion, this absorption hole provides cushion when the protrusion isdeformed (compressed) during the pressing step, whereby chipping of theprotrusion can be prevented. Also, because of this absorption hole, theforce required for deforming (compressing) the protrusion can bereduced.

Furthermore, a manufacturing method of a stator, according to thepresent disclosure, is a method for manufacturing a stator configured toinclude: a main yoke having a cylindrical shape with a bottom; anauxiliary yoke arranged on an outer circumferential wall surface or aninner circumferential wall surface of the main yoke; and a field magnetarranged inside the main yoke and radially facing an outer surface of anarmature. The stator houses the armature secured to a rotating shaft.The method includes a placing step and a pressure step. The placing stepincludes attaching the auxiliary yoke to either the outercircumferential wall surface or the inner circumferential wall surfaceof the main yoke, the auxiliary yoke has a cylindrical shape in which arotatable fastening part coupling one end and another end of anauxiliary yoke body part that is a band-shaped plate member, and therotatable fastening part is rotatable to cause one of the ends to comeclose to or move away from another of the ends. The pressing stepincludes pressing the auxiliary yoke against the outer circumferentialwall surface or the inner circumferential wall surface of the main yokeby rotating the rotatable fastening part to cause one of the ends of theauxiliary yoke body part to come close to or move away from another ofthe ends.

With this configuration, the auxiliary yoke can be easily mounted to themain yoke only by rotating the rotatable fastening part, and similaradvantageous effects as those described above can be achieved.

The stator according to the present disclosure adopts a configurationwhere the auxiliary yoke is mounted to the main yoke. For the mounting,there is no need of press-fitting, welding, drawing, bonding, and so on.Namely, physical impacts of a large force or chemical influences can beprevented. This effectively prevents changes in inner diameter of themain yoke, peeling of coating, thermal denaturation, corrosion, changesby anisotropy, oil accumulation, separation of the auxiliary yoke,appearance defects, and so on. The engaging configuration, or theconfiguration in which the rotatable fastening part is rotated to adjustthe diameter of the auxiliary yoke, is advantageous in terms ofmanufacturing cost because it allows reduction of the accuracyrequirements for the inner and outer diameters of the main yoke andauxiliary yoke.

While the present disclosure has been described with reference toembodiments thereof, it is to be understood that the disclosure is notlimited to the embodiments and constructions. To the contrary, thepresent disclosure is intended to cover various modification andequivalent arrangements. In addition, while the various elements areshown in various combinations and configurations, which are exemplary,other combinations and configurations, including more, less or only asingle element, are also within the spirit and scope of the presentdisclosure.

What is claimed is:
 1. A stator comprising: a main yoke having acylindrical shape with a bottom; an auxiliary yoke having a band shapeand arranged on an outer circumferential wall surface or an innercircumferential wall surface of the main yoke; and a field magnetarranged inside the main yoke and radially facing an outer surface of anarmature housed in the main yoke, wherein the auxiliary yoke is arrangedalong a circumferential direction of the outer circumferential wallsurface or the inner circumferential wall surface of the main yoke, theauxiliary yoke has at least one protrusion formed at one end thereof,the auxiliary yoke has at least one recess formed at another endthereof, the recess facing and engaging with the protrusion in thecircumferential direction in a state where the auxiliary yoke isarranged along the circumferential direction of the outercircumferential wall surface or the inner circumferential wall surfaceof the main yoke; the protrusion engages with the recess in a statewhere the protrusion is in pressure contact with a part of the recess,the recess includes a first hole close to an opening of the recess, anda second hole which is spaced more from the opening than the first holeis from the opening, the second hole communicating with the first hole,a size of the first hole in an axial direction is smaller than a size ofthe second hole in the axial direction, a distal end portion of theprotrusion is compressed and has a spread part expanded in the axialdirection, and the protrusion engages with the recess in a state where asurface of the spread part facing a base of the protrusion is in contactwith a step formed between the first hole and the second hole.
 2. Thestator according to claim 1, wherein a size of an opening of the recessin the axial direction is smaller than a size of an inner part of therecess in the axial direction, a distal-end side of the protrusion ispositioned in the inner part of the recess, and the distal end portionof the protrusion is in pressure contact with a part of a peripheraledge defining the inner part of the recess, and a size of the base ofthe protrusion in the axial direction is smaller than a size of thedistal-end side of the protrusion in the axial direction, and the baseof the protrusion is positioned in the opening of the recess.
 3. Thestator according to claim 1, wherein a buffer hole is formed in at leastone of a vicinity of the protrusion and a vicinity of the recess.
 4. Thestator according to claim 1, wherein the protrusion and the recess areprovided at or near a position radially overlapping with a position ofthe center of gravity of the field magnet.